The changing faces of Fagradalsfjall: fizz, bubbles and slugs

We have had quite a ride. The eruption began unseen, on March 19. The new fissure opened on April 5, after the initial double cone had begun to wane. The new fissures sprouted a series of cones, mostly twinned. By May, all twins had exterminated one of the siblings, and the survivors had battled for supremacy leaving one winner. From now on, this would be a singular eruption.

Remember our adorable cone-let that started the eruption? Now completely buried?

On midnight May 2nd, everything changed. The tremor increased, the eruption went out, and then suddenly the world exploded. This kept happening, as often as 5 times per hour; 400-meter tall fire fountains were visible from Reykjavik. The seismographs remained extremely noisy for a long time, many weeks, while the volcano kept booming and the lava rose and fell around every boom and bust. Over time, the strength of the booms diminished and the eruption became a pulsing one, with a lava pulse (and a boiling lava lake) every 8 or 9 minutes. Amazingly, during all this time the eruption volume remained constant.

There was a general expectancy among volcanologists, volcano watchers and the tourist board that the eruption could continue for years. But on June 28 the eruption suddenly stopped, only to resume a few hours later. This has since become a pattern. The seismographs suddenly go silent and lava retreats out of sight, leaving an empty cone. Then the noise slowly increases, lava rises and a flood of boiling lava appears, looking like a lava tsunami coming down the sides of the cone. The flood diminishes, the seismographs goes flat and the cycle repeats. The duration of the active periods has varied from a few hours to a full day but apart from a lengthening there is no clear pattern. And yesterday the volcano flat-lined and did not come to life again for over a day. Is this the beginning of the end, or is it the end of the beginning? Who knows. GPS measurements show deflation around the eruption: lava is erupting faster than it is being replenished. That can not last forever. On the other hand the remarkably constant effusion rate shows that the eruption is not limited by available pressure but by the carrying capacity of the conduit.

In the mean time, the Iceland Meteorological Office ordered extensive hill fog to blanket the eruption into extinction. That didn’t work. Iceland’s engineers experimented with other types of eruption control. On May 14 they began work to build two walls, in order to contain the lava field. To our amazement it worked for a while but eventually the walls were overtopped. Other walls were build to protect Natthagakriki (June 15) and the coastal road (June 25). These still hold, helped by the fact that the lava gave up and decide to flow the other way. The new walls haven’t really been tested yet. If the eruption now ends, then the engineers will be happy and the government will declare that the battle against the Earth was won and take credit. If the eruption continues, then all bets are off.

The recent eruption has shown four distinct phases, with possibly a fifth just happening

  1. Constant effusion. This was the start when the lava flowed continuously;
  2. Intermittent fountaining. This was an exciting phase, approaching a strombolian eruption;
  3. Pulsing with a few minutes of activity followed by some 5 minutes of quiescence; each pulse produced spattering and mini-fountains, with quiet flows in between;
  4. Intermittent activity, with bubbly lava lakes for a time of several hours or longer with lava flooding, followed by a sudden retreat where the lava disappears and the seismographic first show pulses and then goes dead quiet. After some hours (or longer) the seismograph begins to show a bit of noise, which increases while the lava slowly rises up again. Over recent days the pulsing during the turn-off has become very weak.
  5. (Failure. The eruption interrupts but does not restart)

The amazing aspect is that the average flow rate did not change over this time, at least until the last few days. All measurements have returned 13m3, ever since early May, while the eruption went through these changes. Neither has the lava composition changed much, although there have been some minor variations. What is happening?

Let’s first look at the lava itself. It has a mantle-like composition, i.e. a form of basalt. Basalt may not be what you want in your plumbing, but in fact it flows quite well as long as it remains insulated. It has a low viscosity and is low in silicates (the two are related). The composition has shown that the magma formed at a high temperature of around 1250K at a depth of 15-20 km. It erupts at a bit lower temperature of 1170K. It contains some amount of CO2, and less SO2. This did not change during the changes of the eruption style. The dissolved water content has not been reported but for Iceland eruptions is typically around 0.8%.

The seismograph signal shows the noise that is generated as this magma flows up the conduit that connects to the surface. The signals are shown for high frequency (higher than 2Hz) and low frequency (less than 0.7Hz)

This is an example of the high frequency signal, showing the sudden stop.

Here is the low frequency signal, showing much less of a change.

What causes the noise? First, notice there are no sharp spikes visible. This means there is no rock cracking, which would show up as many small earthquakes. The lack of crackling shows that the conduit (and the dike underneath) is stable. The plumbing in this eruption is now well established and it is not in need of work. That will change only when the magma retreats from the conduit. We do see occasional rock falls on the steep inner side of the cone, giving a drawn-out signal lasting a minute or more. There are none in the plots shown here. Instead, the signal we see comes from magma moving through the conduit.

Magma can flow in two ways. The flow can be laminar, like honey creeping over a surface, or turbulent, like water in a steep river bed. Laminar flows are silent. The honey that is in touch with the surface is almost stationary, and the further it is from that surface the faster it flows. So you never have a fast flow directly over a corrugated surface. Turbulent flows are very different. The liquid is moving at different speeds and even in different directions and there is a lot of interaction between fast flows and surfaces. This seems the noise that we see. When the seismograph is noisy, something is causing turbulence in the flow. When it flat lines, the flow is undisturbed and laminar – or it has stopped.

Two flow types in the magma conduit. The right hand one causes a noisy signal on the seismograph.

Do be aware that the seismographs can pick up noise from other sources. Especially the low frequencies pick up movement over large areas, sometimes including visitors. (They also see large earthquakes across the entire world.) Wind can affect both plots: the plot thickens and becomes noisy. When it storms, eruptions become hard to see.

The signal does not tell us where the flow noise is located. It could be in a deep conduit, it could be on the surface or in a lava tube. If you look hard at the high frequency signal, squint a bit, and use a bit of imagination, you can see a hint of pulsing just before the end, lasting maybe 15 minutes with pulsing repeating over 3 ot 4 minutes. That can only be in the conduit, so I am assuming that the entire signal comes from the vertical pipe through which the magma rises to the surface.

What causes a flow to become laminar or turbulent? The smoothness of the surface is important. A river can be beautifully laminar where it is wide and has a smooth sandy floor, but turbulent where it becomes rocky or changes its width. The viscosity is also important. A fluid with high viscosity (internal stickiness or friction: think honey) tends to flow laminar, while a low viscosity fluid (such as water) very quickly becomes turbulent.

The magma in Fagradalsfjall is a type of basalt which has low viscosity. That is both because of its composition (it has few silicate crystals which easily stick together) and because of its high temperature. The lava channels show a fast flow, and this is indicative of a low viscosity lava. By the time it gets to the end points in Meradalir and Natthagi, it has cooled down and when flowing on the surface it behaves more viscous, although not nearly as much as rhyolite which just refuses to flow and sticks to the ground.

We would therefore expect that our magma can easily become turbulent. It doesn’t in the lava channels: even though the lava flows fast, it is still laminar. That is by and large also true underground, as long as it flows through wide pipes or tubes. Indeed, the seismographs was noisy when magma was still breaking through to the surface and did not yet flow, but they became rather quiet when the eruption was well established. The flow underground was also laminar. But later the eruption became fountainous and after that bubbly. And the noise went through the roof. There was turbulence in the magma. The fact that this happened while the lava was bubbling suggest that there was gas in the plumbing. The gas in the low viscosity magma caused turbulence. Where did the gas come from?

Let’s take a step back, or rather down. The magma rises up because it is buoyant: it has lower density than the surrounding rock. The hotter it is, the lower the density, and so hotter lava (if there is a choice) rises faster. As it rises, the depth becomes less and there is now less pressure from the weight of the rock above. The pressure in the magma decreases. At the same the temperature also drops a bit. To give some numbers, at 15-20 km depth where the magma was sourced, the pressure was around 400 Mpa (4 kbar if you prefer) and the temperature was around 1220 K. (The melt had actually formed even deeper, perhaps 25 km.) By the time it entered the dike, at 6 km depth, the pressure was down to 150MPa and the temperature around 1200 K, and at the point where the dike connected to the conduit, perhaps 2 km depth, they were 50 MPa and 1190 K respectively. The magma erupted at the surface with a temperature of 1170 K.

Source: AGU webinar on Fagradalsfjall: Dr. Eniko Bali. https://www.youtube.com/watch?v=u3QW22b9_Jg

The origin of the gas lies in the changing conditions during the rise. Liquid magma can contain a limited amount of volatiles, such as water and CO2. If there is more of these then can dissolve into the magma, the excess is expelled and becomes a gas – a vapour inside the magma. The maximum amount that can dissolve in the liquid is called the solubility. It is different for each volatile. To give a rough number, basalt at 1200 K and a pressure of 50MPa can contain around 2% (by weight) of water. This amount decreases rapidly with pressure: by the time the pressure is down to 5 MPa (200 meters depth), the solubility is down to around 0.5%. It scales roughly as the square root of the pressure.

Temperature has the opposite effect: as the magma cools it can contain more water. You can see this effect when heating water in a pan. As the temperature rises, bubbles appear in the water. This is gas coming out of the liquid. Let the water cool and the bubble disappear again, taken up by the water. But in this magma the effect of temperature is fairly minor. The solubility of water scales roughly as 1/T (with the temperature T in Kelvin), and the temperature drops by only around 5% between 15 km depth and the surface. The pressure reduces much more dramatically, and it wins the battle. So while the magma rises, it tries to dry out and expel excess water. You may want to think of a volcanic eruption as a giant dryer.

Icelandic magma is pretty dry to begin with, but not that dry. At 0.8%, the Fagradalsfjall magma reaches a problem at 500 meters. At that depth it becomes saturated. As it rises further the magma begins to expel water and water vapour (steam) develops in the magma. The magma becomes gassy, and just like a human body after a good meal, it becomes windy and noisy.

CO2 goes through the same process as water but it does so at much greater depth. Mantle plumes may contain 1 % CO2 by weight, but this already turns into gas at a depth below 5 km. Some of this CO2 finds its own way to the surface and some remains as a gas inside the magma. By the time basaltic magma is at 1 km depth there is little dissolved CO2 left.

So water in Fagradalsfjall’s magma produces vapour during the last 500 meters of the ascend. This is not an easy process. A phase change (liquid to vapour or liquid to solid) needs something to hold on to. Pure water can in fact be cooled to well below freezing while still staying liquid. But shake it a bit and it freezes instantly. The water was supercooled. Air too can be supersaturated whilst not producing clouds . But when a passing plane disturbs it, instantly a contrail forms. It is the same with magma: it can become supersaturated with water but still reluctant to let it go. It takes time for the water to evaporate out of the liquid. If this is longer than the time it takes the magma to reach the surface, then the water will stay in the magma as an unwelcome passenger.

(I remember a camping trip (in Africa!) when after a chilly night we tried to pour water from a bottle into a cup. It froze on the way, mid-air. The water had become supercooled.)

When the water turns to gas, it forms small bubbles inside the magma in a process called bubble nucleation. Initially these are tiny, microscopic even. Nucleation is much easier when there are crystals in the magma: they provide a surface on which the bubbles can grow with ease. If there are no crystals, bubbles form with difficulty and the magma becomes supersaturated. But take supersaturated magma and add nucleation sites (crystals) and bubbles instantly form everywhere. If a magma rises rapidly, it will become supersaturated because the water has no time to respond to the decompression. But as the pressure continues to fall, at some point the supersaturation may become so high that nucleation accelerates anyway. Suddenly, gas is everywhere. The magma becomes fizzy and turbulent.

The bubbles are very buoyant and try to rise. But the magma is too viscous for that. It is worst for the smallest bubbles: friction with the magma locks them in place. Larger bubbles find it easier to rise, especially in a low viscosity magma. Let’s assume that the magma in the conduit rises at a speed of 1 m/s. That is a reasonable value for Fagradalsfjall: it gives the right flow rate (13 m3/s) for a conduit that is 4 meters across. The bubbles will move up a little bit faster, but not much faster. Even in Fagradalfjall they will only go faster by a few cm/s. The magma now becomes a mix of liquid and bubbles.

As more of the water becomes gas, the bubbles grow and take up a larger fraction of the volume. As the bubbles become mobile, they collide and can merge, or take in more water from the surrounding magma. And they also expand because the pressure is dropping as the magma rises. The bubbles can grow as large as a few centimeters. So as the magma approaches the surface, more and more of the volume is taken up by gas.

When bubbles take up more than half the volume, the bubbles merge into gas pockets. These are called ‘slugs’ and they take up the full diameter of the conduit, pushing the magma out of the way. If 90% of the volume is gas, then the slugs merge into a column of gas and magma is pushed to the side, but this may not happen in real volcanoes. In a bubble flow, the bubbles are stuck and rise with the magma. But in a slug flow, the slugs can rise rapidly because of their low density and because their smooth surface pushes the magma out of the way. The bubbly flow is sluggish and the slug flow is not.

What kind of speed can we reach? Here, the change of density is important. If half the volume is taken up with bubbles, the density of the mixture has halved. The magma now becomes very buoyant. If we start at 500 meters depth and use all the energy in the buoyancy compared to the surrounding rock to accelerate, by the time we reach the surface the velocity can reach 100 m/s. At that speed, a ballistic trajectory can reach 500 meters height. This is about what the highest fountains in early May reached. (The reality is of course much more complex. Much of the energy is lost in friction in the conduit and the slugs don’t travel anywhere near that fast. On the other hand the slugs still expand and this expansion greatly adds to the velocity.)

Slug flows are the dominant cause of strombolian eruptions. Each slug, when arriving at the surface, causes an explosion both because of its speed and because the slug expands fast in the low pressure around it. It throws out the surrounding magma (lava?) with it; it fountains, fragments and falls. If there is debris plug on top of the conduit and/or stagnant lava, the slug can become more explosive, and produce ash. If the conduit is open the fragments are ballistic lava. Fargradalsfjall always had an open conduit.

This degassing of the magma was the driving force during the fountaining phase, and the eruption changed because it began to degas much more. Originally, when the eruption first began, the magma did little degassing: this was the time of the constant outflow which we saw coming from the first cone, and later from the fissure. The magma at this time may have been less supersaturated, so that the bubbles formed slower and never merged into slugs.

There can be several possible causes for the change to slug formation. The magma may have changed, and had a higher supersaturation. This was also the time that the flow rate increased to its current value of 13 m3/s: this is possible with the same conduit if the density or viscosity of the magma became a bit less. The change allowed for faster bubble nucleation.

The eruption became extremely noisy at this time: because of rapid degassing the flow became bubbly on the ascend and therefore turbulent. The whole conduit, from 500 meter down to the surface degassed together. I envisage this as starting near the top. The sudden appearance of many bubbles drives out magma, and this reduces the pressure lower down where the magma now carries less weight. This decompression increases the supersaturation and allows bubbles to form here, and so on. A bubble formation front accelerates downward and the whole column turns first fizzy and then bubbly, before the rising bubbles begin to form slugs. It is just like opening an overpressured bottle of carbonated water.

(No slugs were harmed (or produced) in the making of these movies)

(An alternative idea is that the seismograph noise that we see comes from the bubbles themselves, as they implode, explode and merge, so that the turbulence is not in the flow but in the bubbles.)

Why the 10 minutes between the strombolian fountains? At a speed of 1 m/s, it takes ten minutes for the 500 meters of degassed magma to be replaced by the gas-rich magma from below, after which the process could repeat.

The pulsing that happened later was different. There were no high fountains, and there was no large acceleration. There were no slugs. Also, the seismographic noise happened mainly during the pulse whilst in between pulses the flow was much less noisy. The magma now was less supersaturated and fewer bubbles formed. The same process as before happened but the bubbles never reached the slug threshold of 50% by volume. The magma became bubbly but not sluggy. The bubbly flow still reduced the density of the magma, and this caused the level of the lava at the top to go up. The lava lake filled up and overflowed. The lava degassed over a couple of minutes as the gas reached the surface. This caused the apparent boiling of the lake. Once the gas became depleted, the density of the lava became higher and the level of the lake went down again. The lower noise of the seismograph now showed much less turbulence from bubbles. Over the next 5-10 minutes the magma in the conduit was replaced by fresh magma, and the degassing would start again.

This process explains why the flow rate of the lava did not change. The rate at which magma ascended from below remained constant all through these phases.

The current phase of long quiescent and active periods is different again. The fact that the seismographs are flat during the quiet period suggests that the flow becomes laminar without bubbles. There is no gas: the magma is not supersaturated. The density is therefore also higher and the level of the lava lower. We don’t know whether there still is any lava flow during these periods but it is possible it still flows -silently- through a deeper tube towards Meradalir, below the layer of the rubble that we can see in the cone. However, it is also possible that the magma in the conduit stops flowing and the eruption interrupts. It depends on how buoyant the magma below the conduit is.

Slowly the low frequency noise increases. A bit of degassing is beginning but the volume of the gas remains small. As the degassing increases the density of the conduit decreases, but only a little because there is less water available for degassing. The magma begins to rise, and the pressure below begins to drop. This causes some supersaturation and slowly more gas comes out of the magma and forms bubbles. The density decreases as before, the lava level rises and the cone overflows. The process is slow enough (several hours) that the magma is always in equilibrium, able to shed the excess water and avoiding supersaturation. The new situation with the magma a little bubbly but not very much is stable: it will continue as long as nothing disturbs it. But a rock fall at the top or a slowing of the overflow will increase the weight below, and the bubbles begin to dissolve into the magma. There is a little bit of pulsing at this time, but with a shorter period of a few minutes. This suggests that the gas formed only higher up in the conduit. This is of course what you would expect if the is less water in the magma: the depth at which it become supersaturated becomes less. Once the gas is gone, the situation is stable again.

The volcano therefore can have two very different modes which are both stable: one with fizz and one without.The change from one to the other is unpredictable. In physics, this can give rise to a chaotic system with periods of constant behaviour followed by a random large change. If you are interested, look up the Lorentz attractor.

The changing eruption does not necessarily mean that the flow rate has decreased. However, the supersaturation of the magma is changing. Perhaps there is less water. There may also be a more prosaic reason. The eruption has added a lot of weight to the area. The pressure in the conduit is therefore a bit higher than before, and this can reduce the supersaturation.

What will happen next? The current situations is unlikely to last long. The long periods without lava suggests the eruption is on the edge, and could easily stop completely. If that happens (not unlikely), the conduit may block, the walls will survive and the government will declare victory and call elections. But if there is still flow from the mantle, then the pressure from below will increase again and the magma will look for another way out. In that case, after a while the rock-breaking earthquakes will restart and the eruption may eventually resume in a new location.

This may well be the end. It may also be the start of something new.

Albert, July 2021

463 thoughts on “The changing faces of Fagradalsfjall: fizz, bubbles and slugs

  1. There is a very interesting phenomena taking place. I watched the 4:10 am gushing episode. First the lava stream got higher and higher until finally it overflowed from the cone. Then the cone became active and rose up gushing. About 1/2 or 2/3 the way through, all of a sudden the flow to the side clamped down and stopped. Even the lava stopped flowing in the channel to Meridalir. But then the gusher seemed to exit about the backside to the right, there appears to be a portion with the cone caved in and missing a segment and there was a large push of eruption gases and some lava to the back.

    This is the 3rd time I have seen this behavior. Are there two big crevices in the cone now? Why is the clampdown on flow occurring (or seem to occur in the camer) I tried to record this using VLC but I am not sure that I got it to record

  2. it took me a while, but now the video from my hike around the volcano is available:

    https://youtu.be/DgUxObA8BMY

    In the morning of that day, the volcano was declared “done” by chad, but over the course of the day and the hike it became active again.

    The total distance was 24.24km, the hike took me over 15h, though only 6h were I was actually moving.

    The drone is still fine, even with thte battle scards it got from the heat.

      • For photos, post a direct link to the .jpg with a full url

        For videos, youtube have an “embed” option – right-click on the video to create the code

        Not sure how it works for videos hosted elsewhere.

      • What a day, what a journey! Fascinating to see the ground conditions around the valleys, and those slopes must have been tough walking. The music choices are very complimentary. One or two shots that were so quick I couldn’t orient where the camera was. Superb slow work over the braided streams flowing from the tube breakout.
        I particularly liked the pan over the other craters, Ugly Sister cones to the Cinderella turned Princess.
        Seeing a good slow look at that curvy area on the eastern cone side was good too, as that’s roughly where the big breach seems to be. That eastern rampart is so steep, but thin, with that boiling pot looking to be undercutting into the slope (or maybe the small rockfalls, then swept away by the active lava below, are thinning the cone?).
        Thank you for this, definitely in my top 3 videos for the whole eruption.

        • yes, the walking was slow and tedious, but as it doesn’t get really dark I was not in a big hurry.

          The guy with the Mavic 2 Pro who started from the wall protecting Grindavik on Path B didn’t see any lava in the crater. Later when I was behind the volcano there was a bit of fountaining.

          Later from the east of Meradalir, Storihrutur and finally Launggirhykur the views of the bubbling lava and the huge lava streams were mesmerizing.

          When hiking the northernmost finger of the lava field in Meradalir I suddenly got a strange feeling, realizing that everything turned red. Quickly I found out that the eruption cloud of the volcano obscured the sun, creating an eery red cast. Of course at that point the volcano was not visible to me, so I had to wait (and walk) until it became visible again.

          btw: I have also found a shot from behind the volcano in the direction of the eastern exit of Meradalir. The exit is marked by yellow marks (on the ground, not on the picture) I assume they specify the height as measured from the exit. I will post this image later.

          On top of Launggirhykur the next day I found a piece of Audio Equipment, that stilll was not reclaimed a few hours later. I wonder if anyone reading this is the owner of the piece. If you can prove that this is yours (by describing it in detail) I will send it to you.

    • The best footage of the lava flowing down into Natthagi I have seen, really shows how fast it moves when it is hot, probably faster than anyone could run in that terrain.

      • it was fascinating to watch the lava come to a stop at one point, slowly build up and then breach the dam and pour down into Natthagi.

        I am still wondering what caused the intense burning and steaming, it was also clearly audible, and went on for half an hour or so.

        Walking the ridge from Launggirhykur towards Natthagi, the heat of the lava stream below was felt even with the multiple layers of clothing.

        • It must be pretty wet, it is a small gulley and it does get a lot of condensation. Perhaps there was a puddle. Or maybe a bunch of plants were bulldozed by the lava and got stuck there.

        • Does also remind me of those bubble bursts on ocean entries in Hawaii, I think that is how pseudocraters are made. Must have been a real sight to be at Myvatn 2200 years ago, to see this but 100x and all over the place

  3. Finally! I waited long enough for this moment, having posted about it above and wondering about clamped lava flow.

    This video clip I captured from the mbl.is camera at just the right moment, when they panned the camera and went into telescopic view. You can clearly see that the stream is coming from a crack in the crater wall, somewhat high up and eventually the lava level from the lake drops enough to shut the flow off. Furthermore you can see the active lake boiling and spurting lava into the gap in the back right hand side of the cone from this view. So I got all my questions answered. Please see https://drive.google.com/file/d/1-7TVBw33GOQ4l-z_NbIT5WnT_Fkw5nvc/view?usp=sharing

    This has been going on for a few hours now, I am glad that mbl.is panned the camera as I was recording.

    • I’ve watched this several times. Great catch. There’s also a good glow behind the ridge, would that be a second flow, that gets cut off earlier, if it starts higher?

    • I like the men gently approaching it on the yellow mobile crane.
      “Careful, lads! Careful now….gently does it…”
      KaBoom!!!

  4. It was noted last night that part of the come was moving back and forth on the photos. I have tried to capture this in a timelapse – sorry, a bit grainy. Watch the triangular, tooth-like rock in the centre, on the right of the cone. Several times during the time lapse (1900-2300 last night) you can see it move. At one point the whole rock is lifted up by the lava underneath, not so much a lava boat as a lava ocean liner. As soon as the lava retreats, the rock comes down to earth. This bit of volcanic real estate is in danger of losing its mooring.

    • RE:”This bit of volcanic real estate is in danger of losing its mooring.”

      Losing its baby teeth so to speak.

    • Clearly seen that the volcanic edifice is not built according to the building legislations. A civil engineer would have noticed that the walls of this volcano are not stable enough to withstand a sloshing lava lake inside the crater.

      Volcanic edifices are a quite example of jerry-building (Revolutiebouw in Dutch), think about Surtsey’s lava shield which was built upon a foundation of loose tephra. By this way of building the lava shield is easily eroded by wave impacts.

    • I’m thinking about the bench built by the lava from Pu’u O’o along the coast, and how that all collapsed leaving the fire hose lava tube suspended 80ft up in the cliff edge.

  5. Being a total neophyte volcano watcher, I was wondering about the strength of this spatter/cinder cone, especially as it was built on a steep slope. These past couple of days have answered my questions quite dramatically.

  6. The flow has really slowed down with less total volume in the past 60 hours than in the first 24. Meradalir has not gained much more than a million m^3. The flows being on the surface and in many channels has resulted in significant accumulation along the path from the cone. Last night, the northern part of the ridge between there and the old Meradalir channel was overtopped in several places. A half million m^3 or so might have accumulated there. The average flow rate from the cone toward Meradalir is not more than 10 m^3/s.

    • Interesting! The immediate question would be if it is still ~12M m^3 per hour averaged over all the pauses, or now a lower rate overall.

      Where did you get these numbers, by the way? Are there more measurements available outside of the 3D volume analysis from the overflights?

      • Those are just my numbers. Sorry this is a little long!

        I look at Meradalir camera and get two images, one at the start and one at the end of the time period. I then find identifiable features and compare the lava’s progress. A mixture of peninsulas/elevation features (identified on elevation maps) and different colored zones such as minor gullies (identified on Google Maps than elevation maps). That lets me know the elevation within a meter or two as well and the change. I use Google Maps to measure the surface area (all of Meradalir is roughly 1 million m^2 for instance) and multiply by the depth.

        For instance the area from the cone to the steep drop to Meradalir covered in new flows is ~600X200 meters or 120,000 m^2. The top of the ridge overtopped is the 220 m mark and before the recent surge the flow was nearly 10 m below that. 60 (now 65) hours ago it was still a decent bit below the ridgeline, maybe 5 meters or a little more. That would suggest 600,000, plus the gully, plus the volume that flowed down the far side. However, the new area covered at the edges will be thinner, so went with 500,000+.

    • Incredible!

      The video also shows some sort of wave of pahoehoe lava resurfacing, that starts around 1:15, and then moves across much of the lava lake filling Meradalir, until it disappears behind the smoke. Looks like some sort of gigantic foundering event of the lava lake, that was set off by the lava flow going into Meradalir.

      This eruption just keeps surprises coming.

  7. On the mbl close up camera, you can see the floaty lava-berg pop up at 16:06, then sink down again about 2 minutes or so later. Comes back up again later.

    • It’s not going down quite as far this evening.
      How far is this ‘tooth’ moving? And why isn’t it collapsing?

      • It’s still rising and falling at the same rate. I assume it is fairly pyramidal in shape, so quite stable, and perhaps the split at the base is angled up (away from the cone) so slides back down when the pressure drops. Something is bound to give eventually though.

        • That would be a mortice form…wider at the base, tapering towards the rim. As it rises it locks. To keep it from tipping out, it would have to be wider internally than externally.

      • Looking at the mbl.is closest webcam, at 10.30am it couldn’t be seen at all. 16.10 it almost sank out of sight, but between many of the pulses there’s been plenty to see.
        Now, at gone 10. 30pm GMT, it’s staying up pretty well.

  8. I believe that I might have recorded some of the fastest moving lava on earth. [qualifier: I am not talking about Strombolian eruptions or fountaining but regular lava flow] During the 18:04 gushing episode, chunks of lava flew off the sides of the crevice and you can see them flying through the air at high speed. I am guessing at least 60 km/hr. Please take a look around 18:04 pm in the video to see these chunks flying along. I contacted RUV.is people to ask how far the camera is from the cone, if they give me a ball park figure in meters, I can easily calculate the transverse speed of these chunks, and we can see how fast things really went. Please take a look https://drive.google.com/file/d/1zAH09i9xUH2JSC7INuASghN05Y34LxR6/view?usp=sharing

    This is really very fast moving lava. If RUV.is gets back to me and gives me the measurement, I will post the approximate velocity of these lava chunks.

    • I’d hazard a rough guess that flow is running somewhere around 70mph. Just a guess.
      Hope you get your answer!

      • Not sure it is that fast, it is flowing under gravity so theres a limit I think. But very fast lava for sure. I think often lava looks like it is flowing faster than it really is because it is so heavy and has so much momentum and a much higher viscosity than water even in eruptions like this, so it can easily back up into standing waves or ride up slopes. The lava channel in Hawaii in 2018 looked just like this but it was only flowing at 30-40 km/hr as measured, it might have been as much as 60 during big surges but nothing close to 100.

        I would not be entirely surprised if the infamous lava flows at Nyiragongo in 1977 were exaggerated too and nowhere near 100 km/hr advancing speed, in reality lava flowing at 1/50 that speed is likely to take lives in an eruption like that, the recent eruption in most regards was nearly a mirror repeat of 1977 and nowhere was it reported a flow speed that high.
        I think we have a bad habit of overestimating how fast we actually are, you can sprint at 30 km/hr but you cant sprint for 30 km or for 1 hr… And you cant sprint in a forest at all.

        • Note that the speed of the flow and the speed at which is advances are not the same. In a viscous fluid, the bottom layer is slow and the top layer fast. What you see moving so fast is only the top meter or so. The speed of the lava underneath was visible in the past when we still had lava boats in the channels: those boats went much slower, because they sit in the deeper layers. The advancement of the flow comes from lava in touch with the solid ground and that will be slower. It can still be at a decent speed especially on a slope.

      • RE: “I’d hazard a rough guess that flow is running somewhere around 70mph.”
        At that speed the fool who climbed the slope of the cone would have become a modern day ‘Nupondi’ [think ‘One Million B.C.], immortalized in a Youtube video entitled ‘Village Idiot”!

  9. Looking at this webcam (http://brunnur.vedur.is/myndir/webcam/2021/07/13/webcam_meradalahnukurSV.html# – sorry, don’t know how to post a link correctly, so this may end up in the dungeon! Home-made biscuits duly proffered to relevant Dragon…) on running today’s sequence, in the early morning there actually looks to be fountaining, quite high, more than once, where the “leak” comes from – quite some way from the main cone, but at the same time as that’s fountaining. It’s rather higher than I’ve seen any of the previous satellite vents in the lava field produce. Curious as to why?

  10. It’s amazing watching the back right wall float up and down during the pulses on the mbl.is camera! I can’t wait to see the timelapses tomorrow.

  11. Seriously there’s a women face in the cam for the past 1/2 hour .Nothing wrong with a wave or whatever, but stooping to exhibt one’s ego in a selfish pursuit sends me crazy.

  12. On the visir webcam the lava pulses now send a gush of lava towards the front again, which ultimately will build up the cone towards theatre hill or flow into Geldingadalir (what’s left of it). It’s only a brief gush, but as the cone rebuilds on the other side, this could become more substantial with time.

    https://www.visir.is/sjonvarp/beint

  13. I’ve started viewing the Krisuvik tremor plot instead of Fagradalsfjall because the former seems to filter out just the right magnitudes to where you can see whats going on

  14. Tremor has obviously returned strongly.
    So, some youtubers are putting up images of the tremor graphs to predict activity.
    Their explanation of the graph is that the 2-4 Hz lines maximum values coincide with more activity in the crater. For instance, one explained that the 2-4Hz went from high values of ~4000 to ~5000, and interpreted that as meaning the crater would be overflowing a lot more when the peaks were at 5000. This movement from peaks around 4000 on the Y-axis to 5000 meant an increase in activity.
    I know little about interpreting tremor graphs (other than when they are a sustained wide range signal they indicate ground vibrations from magma movement), but that made me laugh as I assume there isn’t a direct correlation.
    I could be wrong though; please correct/forgive my ignorance.

  15. Regarding the moving block of solid lava that was discussed earlier, I noticed that it was obvious in the MBL closeup camera yesterday. It appeared as a background “tooth” to the right of the main crater, in a shallow depression between two higher points in the foreground. The “tooth” rose noticeably during the active phases of boiling lava and dropped down between those times. What is interesting is that the “tooth” has dropped down further today so that it is no longer visible during quiet periods. A good example can be seen at 2021-07-14 13:43:00, when the “tooth” is at its peak. By 13:45:00 it is gone. The Weblink is https://www.youtube.com/watch?v=tuLhOThj3Yg.

    • Beware: the tooth is at the front of the cone. It is seen when smoke inside the cone obscures the back. As soon as the smoke is gone, the tooth is invisible against the backside of the rim. It is always visible when lava inside the crater lifts the tooth above the rocks in the back, but when it is back in its rest position it depends on crater obscuration.

      • I don’t doubt that the tooth is in front of the main cone, but in the MBL view it is to the right of the cone and behind the ridge of rock that is part of the old main outlet channel. Yesterday the tooth was visible regardless of whether the lava was active or quiet, but today I watched it sink out of sight as the activity died down. The point that I was making is that the tooth is sinking overall.

          • Thanks for that link. I was using an earlier link to that camera until all of the MET cameras were closed to the public. I didn’t know that there is current access to more than just the Meradalir camera because none of my old links was working. However, I noticed that your link is the same address as my old link; and, after fooling around with the old links, I found out that Chrome had somehow designated them as HTTPS addresses, which they are not. By removing the S in HTTPS, I can get them to work again.

            Your link does show the whole tooth when the lighting conditions are right, but it was easier to see movement in the MBL view due to the juxtaposition of the tip of the tooth near some obvious adjacent features. It’s too bad that the tooth will probably sink completely out of site very soon in the MBL view.

  16. The lake looks like it is gone and everything is just gushing out the side of the crater now

    • Where’s the lava going then – and what camera are you seeing that on? Because I can’t find a camera with a decent view – there are two close-up cameras – visir and the mbl one, and sometimes the ruv one when it’s not pointing at the currently useless wall – but the mbl panorama camera is stuck on 10/7/21.

      I wish they’d just put the Storihrutur camera back where it was. That was the best view.

  17. I worked out a crude estimate of lava speed from the leak on the active cone using the video
    https://drive.google.com/file/d/1zAH09i9xUH2JSC7INuASghN05Y34LxR6/view?usp=sharing

    It is very hard to get accurate measurements of the active cone size. I approximated 170 meters for about 1/2 the way up the visible cone from the RUV.is camera. Using kdenlive, I looked for traveling lava chunks and found one moving at a good rate (but matched by the lava underneath). It took 21 frames at 29.97 fps to travel 25 pixels (measured by gimp) while the cone width was 289 pixels. Using 289 pixels -> 170 meters, I used (170*25/289)/(25/29.97) = 17.629 meters/sec –> 63.5 km/hr. This is a bit faster than my estimate of 60 km/hr.

    I used frames 2044 and 2069 to make this estimate [26 frames = 25/29.97 secs]

    63.5 km/hr is very fast moving lava, even by Iceland standards

  18. The eruption mode seems to have changed slightly. I am watching an episode power up around 23:14 pm. But what is different is that an area in the center of https://www.youtube.com/watch?v=BA-9QzIcr3c seems to power up first, releasing big clouds of volcano smoke, before the active cone finally sets in a while later.

    Question: the bubbling of this lava pond seems to indicate degassing occurring, but could that liquid be coming from the active cone? It is puzzling, if from the active cone, why it wouldn’t be degassing there first, before doing it in the pond to the right.

    I noticed a lot of spattering from the yellow hot lava.

  19. To me, it looks the eruption activity is now very similar to the apocalyptic overflows from last month, except that they don’t flow towards the camera now, and the lava lake may be at a lower level than before.

  20. Full blown gusher now, exiting the central cone pond to the right, looks like the wall has finally been completely breached, thousands of tons of lava now headed for Miradalir valley, won’t take long to fill it up now.
    Time: 01:08 am

    • Thanks for the timestamp – pretty impressive. Pity there aren’t any live cameras out there. 🙁

    • Seems the end of another pulsing episode, 5 days this time. Tremor plot in the “quiet mode” now.

      How long before it starts again… 3 days? 😁

      • The tremor stopped after the big lake drainage but before the collapse. The collapse was (me thinks) because of the emptying of the lake and the episodes ended (me thinks) also because of the lack of a lake. The cone sprung a big hole, emptied, and this changed the dynamics of the eruption. Nothing will happen (me thinks) until enough lava has been raised for a proper refill.

        • I wonder how much of the wall is left. When the lake outflow threshold of the cone wall is lowered much, the lake (or what is left of it) is less deep, the surface closer to the conduit.

          A less deep lake would smother the upwelling, degassing magma not that much anymore.
          Likely a next episode shows higher fountaining again.

        • Albert, in what upper part of the dyke would the major part of degassing take place?
          Upper 100 m? Upper 10?

          • My guess would be in the upper few hundred meters. But it is a guess

  21. 06:07:13 – some type of collapse occurred in the central cone as lots of brown smoke came up

    • You can see this collapse on the FAF seismo

      I wonder if the small glitches a few hours earlier is our toothed crag moving slightly?

  22. The Visir close-up camera appears to have stopped at 23:10. Meanwhile the Langihryggur vog-fogged cam appears to have become a “fool’s camera” for all and sundry to goof around and act like idiots. An hour long festival of gloved finger-puppetry, knitted hat-wearing fashion shows, and gurning contests is doing Iceland’s reputation no favours among the international community.
    (Sigh. Grumpy old man needs coffee!)
    RUV need to pop that camera on a pole, or something.
    And don’t get me started on ‘bum cam’…

    • As the eruption gets more popular, so does the number of working cameras diminish!

      The tremor is way down now so presumably the crater’s having a rest.

    • Some of the posts on Facebook are inane to the mind of a scientist. Give up on your frustration with the tourist mindset regarding this event. I fear its being promulgated as much by local interests as by international lay curiosity. A ‘do no harm’ volcano access to which requires no more than the ability of a seasoned trekker for which there is no dearth of in the physically conscious young. This is not Ambrym on which, a couple of idiots, looking for a lava lake, recently learned respect for Mother Nature. Perhaps you can persuade the local scientific community to establish a dedicated video resource apart from entertainment industry to monitor this ongoing phenomenon.

    • Also trouble in Belgium and the Netherlands… the flood is making its way down the rivers. Got a call to assist, but I am on my way to the airport.

      But the dirst images of germany reminded me directly of the article Albert, very interesting read and aperently still lessons could be learned..

      • Hopefully, the ‘ruimte voor de rivier’ (space for the rivers) program yields success! Delta works 2.0 being tested these days!

    • The River Rhein at the City of Cologne has risen 143 cm within 24 hrs !!!
      till tomorrow evening it is expected to rise another 100 – 130 cm to about 8,38 Meters.

  23. A timelapse without people showing off from the Langihryggur camera.
    Cutting out the people does also mean that some surges were cut out unfortunately.
    The crater wall is clearly moving, which I mostly noticed due to Albert’s earlier observation.

    The timelapse is from 21h45 yesterday to 06h15 this morning.

    https://youtu.be/WsaCBvW9RVo

  24. You can watch the toothed-crag slowly sink away in this time lapse video https://www.youtube.com/watch?v=WsaCBvW9RVo from about 5:40 am and beyond (this is on the far right of the video timeslide)

    The crag slowly sinks into the mush, the next 27 mins or so, with much volcanic smoke being given off, until a collapse in the cone itself around 6:07:13 am or so. It eventually disappears from sight of the RUV.is camera

    • The litlihals camera has moved to a new feed. it shows a new wall being build. not sure where

    • RE:”Looks like they moved the close up camera to Meradadalir.”

      Anything to keep those ducks in ” ‘Made it Ma!’ ‘Top of the world!!’ ‘Do you see me!?!?!’ Club” off the screen.

      • The Litlihals camera show on-going work. It seems a cable is being laid. Electricity supply? Fibre internet? Ground heat pump in advance of the lava?

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